1. Field of the Invention
The present invention relates generally to audio drivers and specifically with DC offset cancellation.
2. Related Art
FIG. 1 shows a conventional digital audio driver. Driver 100 comprises a digital to analog converter (DAC) 102, amplifier stage 104 and an output stage.106. DAC 102 converts a digital audio signal into an analog audio signal. Amplifier stage 104 amplifies the analog audio signal. The primary purpose of output stage 106 is to maintain the output regardless of the current drawn through it, but in some implementations the output stage may supply some amount of gain.
In most analog circuits, such as the audio driver, a DC offset is present. FIG. 2A schematically shows the DC offset as fixed voltage 202 in series with an ideal audio driver component 204, which provides audio output 206, referenced as VOUT. This output is used to drive load 208 represented here by headphones.
As shown in FIG. 2B, issues with a DC offset in an audio driver are often resolved where the load 208 is AC-coupled, such as with external capacitor 210 which is inserted between the output of the audio driver and load 208. However, modern integrated audio drivers consolidate components onto a single integrated circuit eliminating costlier components such as a large capacitor. Larger capacitors provide better isolation while minimizing the affect on the frequency response of the audio driver. If a smaller capacitor were used, there would be more attenuation in the low frequencies resulting in perceived diminished “bass” by a listener.
Integrated audio drivers that are directly coupled to the load introduce problems due to the presence of DC offset at the driver output. The DC offset will cause current to flow into the load even when no sound is present increasing the average power consumption. This is particularly a problem when driving low-impedance headphones. Another problem is that when the driver is enabled, the output voltage will abruptly change, causing an audible “pop” sound.
FIG. 3 illustrates the pop problem in direct coupled audio drivers. Graph 302 is representative of the enable signal received by the audio driver. Graph 304 is representative of the output voltage of the audio driver around the time the audio driver is enabled. At time 306, the output voltage jumps from 0V to VOS, resulting in an audible pop even for offset voltages as low as 1 mV.
Traditional techniques for removing DC offsets from amplifiers fall into two categories, auto-zeroing and chopper stabilization. FIG. 4 illustrates an analog method of auto-zeroing applied to an amplifier. When using auto-zeroing the amplifier has a sampling phase where the DC offset is removed and an operation phase where the amplifier amplifies an input signal. In the example shown in FIG. 4, switch 410 is connected to ground, or in the case of a differential amplifier, the positive and negative inputs of the differential amplifier are connected to each other (i.e., a zero input in either case). Offset voltage 402 and amplifier 404 produce an output which should be representative of the DC offset because the input is effectively zero due to switch 410. The output is sampled by sample/hold 408. Optionally, buffer 406 is used to supply the voltage to sample/hold 408. Buffer 406 may be an amplifier with or without gain. The purpose is to sample the voltage at the output without supplying a significant load which could affect the output voltage. Sample/hold 408 feeds back the voltage to amplifier 404, which is adjusted to zero out the offset. After the sampling phase is completed, the adjustment to the amplifier is fixed and switch 410 connects the amplifier back to the input and the amplifier functions now in the operational stage.
FIG. 5 illustrates a digital implementation of auto-zeroing applied to an amplifier. Switch 512 operates similarly to switch 410 in FIG. 4, controlling the input to the amplifier when in the sampling phase and the operational phase. The remaining components are similar to their counter parts in FIG. 4. The key difference is successive approximation register (SAR) 508 is used to determine the voltage after a number of clock cycles. The digital representation of the voltage is fed to DAC 510 which supplies the voltage to the amplifier so that the voltage can be adjusted to a zero offset. Once the offset voltage is determined, the SAR output can be fixed and the amplifier transitions to the operational stage and functions with zero offset.
The chopper stabilization approach applies a modulation to the input signal and a corresponding demodulation to the output signal. Since the DC offset only encounters the demodulation, it is effectively modulated to a higher frequency. More specifically, FIG. 6 illustrates a basic chopper stabilized amplifier. Again the amplifier is shown as ideal amplifier 604 with fixed voltage offset 602. The input signal is modulated with mixer 606 where carrier signal 612 is the desired frequency that the DC offset is displaced to. The output signal is demodulated with mixer 610 with carrier signal 614. Typically the carrier signals are square waves at a given frequency. Because the input is modulated and then demodulated, its frequency profile does not change. However, the DC offset is effectively modulated by mixer 610 to the frequency of 614. In this fashion, the DC offset is removed from the amplifier.
A drawback of the auto zeroing approach is that in an audio driver the load may be connected to the driver before it is enabled. When the amplifier is first enabled, the offset would be present because the auto-zeroing has not been applied to remove the DC offset, and a pop would still be heard. One attempt to remedy this is to use two amplifiers and “ping-pong” between them. That is while one amplifier is in the sampling phase, the other amplifier is in the operational phase and only the one in the operational phase is connected to the output. This can prove costly to implement because it doubles the hardware used in the amplifier stage and, furthermore, it may not solve the problem because neither amplifier can be auto-zeroed until they are enabled, so there may still be an initial pop.
A chopper stabilized amplifier would have no DC offsets from the time it is enabled because there is no sampling phase like the auto-zeroing technique. The drawback of the chopper stabilization is that it is a more complicated solution. Generally the chopper stabilization is used only to remove the offset from an amplifier leaving potential DC offsets from other components in the audio driver. Even though the DC offset is modulated, it still is present except as a higher frequency signal. Unless this signal is filtered out, it can still cause current to flow to the load.
FIG. 7 illustrates another approach to isolating an amplifier until the DC offset can be removed by auto-zeroing. For the sake of example, the auto-zeroing technique of FIG. 5 is used. During the sampling phase, switch 702 disconnects the audio driver from load 704. After the sampling phase is completed switch 702 is closed and connects the audio driver to load 704. The switch 702 is required to switch a signal after it has been amplified and can carry substantial power. Some drawbacks of using such a switch are that the switch can be very expensive or/and degrade the driver performance.
There is a need in the industry to eliminate DC offset from an audio driver which is not expensive and does not degrade the driver performance.